When doing electrical work, being able to identify wires by their color coding is an essential skill. Grey wires mean different things depending on where you are working or on where the wire or device was produced.
In the U.S.
In the U.S. AC system, grey wiring is not one of the "common" wire colors. Instead, it is a federally-accepted alternate for the neutral wire, one whose principal color is typically white. This color-coding system is required by the U.S. National Electrical Code.
In Europe, wiring colors are overseen by the International Electrotechnical Commission. Their standard grey wire is the "Line-phase 3" in its AC code. In DC circuits, a grey wire is the negative. Current to 2010, the United Kingdom employs these European regulations. There is no common grey wire found within Canadian color coding.
Other AC wire colors in the U.S. code include bare, green or green yellow for the ground or protection wire, black or red for the single phase, and black, red and blue for additional phases. U.S. code does not recognize any grey wire in a DC circuit, although it has no formal recommendation for the negative or positive.